Universal access to telephone services has become a recognized necessity in society. It is regarded as an essential service for the elderly in particular. Those members of society who have had to contend with a hearing impairment, whether by reason of age or otherwise, had less than fully satisfactory access to telephone services. Unless the telephone handset had a built-in amplifier, depending on the degree of hearing impairment they often could not use it.
In the United States it is estimated that over sixteen million people are hearing impaired, seven million of whom are over sixty five years of age.
Although there is no agreed on international standard of classification, a practical guide to hearing impairment is found in the following four categories.
(1) Moderate precipitous Impaired: Have mild threshold loss between 0 and 25 dB at low frequencies from 250 Hz to 1 kHz with a precipitous drop of hearing loss from 2 kHz to above 4 kHz with a slope usually not less than 25 dB/octave.
(2) Moderate Gradual Impaired: Have threshold loss between 10 and 40 dB at 250 Hz with a gradual slope of 10 to 20 dB/octave to about 4 kHz.
(3) Severely Impaired: Have threshold loss of between 40 and 85 dB in the frequency range up to 4 kHz with no threshold differences higher than 15 dB between any two frequency octaves within that range.
(4) Other Impaired: Have threshold loss, usually selective by frequency, which does not come within any of the above categories.
The most common condition in which a hearing aid may be required is that condition known as "sensorineural" hearing loss. It results from damage to the nerve centres in the inner ear, the nerve pathways to the brain, or possibly that portion of the brain that receives and interprets audio signals. It is characterized by the inability to hear sound in specific frequency regions or in a whole audible frequency range, which may lead to difficulty in understanding sounds present in normal speech. This inability is detected as, for instance, a shift (upward) of hearing threshold. It is also accompanied by change of dynamics in speech perception. Accordingly, this condition is alleviated by hearing aids the frequency responses of which are tailored to the impairment characteristics of the individual user.
The function of a telephone hearing aid would normally be to couple its output, acoustic and/or magnetic, to the hearing aid worn by the user, who places the handset to his head in the usual fashion. Since the worn hearing aid compensates for the unique impairment of the individual, a telephone hearing aid should yield a uniform frequency response with some gain. The telephone handset itself, however, only gives the proper acoustic output when properly loaded, i.e. when proper sealed coupling exists between the receiver cap and the pinna of the ear. Thus, "loose" coupling of the telephone receiver to a hearing aid impairs the standard characteristic response of the receiver. This is generally speaking often the case, even with so-called in-the-ear hearing aids, which are often not sufficiently in the ear to allow a good seal between receiver cup and pinna.
Apart from acoustic coupling, some hearing aids have magnetic pick-up capability, which either responds to the magnetic field inevitably generated by most telephone receivers or to magnetic field generated by an auxiliary coil built into the handset for that purpose alone. The worn hearing aid may then, if it is so equipped, be switched to a "telecoil" position, where in most cases its acoustic input is disabled. There seems to be evidence, however, that even with magnetic coupling it is desirable to have an acoustic signal directly transmitted through the ear mold of a hearing aid, even when the mold somewhat occludes the ear canal, to improve comprehension.
As may be gleaned from the above discussion the problem is somewhat complex. The problem is compounded by lack of qualified analyses of the parameters involved, subjective as well as objective. Lack of unformity of hearing aid design also causes inconvenience. A brief exposition of hearing aid types in common usage is in order to facilitate understanding objects and features of the present invention.
It is now in order to discuss the technology of hearing aids proper. The hearing aid is an electro-acoustic device detecting sound from the environment, amplifying this sound and finally delivering this amplified sound to the ear. Conventional hearing aids are classified in power categories by amplification, such as: very strong, 65 dB or more; strong, 56-64 dB; moderate, 46-55 dB; mild, 31-45 dB; and very mild, less than 30 dB. There is no agreement on correctness of classification by gain. Consideration of frequency characteristics leads to another classification, such as: wide band; high frequency hearing aids; etc. There are four common hearing aids available on the market when classified by their construction. The smallest and most inconspicuous is worn completely in the ear ("in-the-ear" type). It provides limited amplification and is best suited to people who have only a mild hearing impairment. The largest and most powerful aid is worn on the body with only the receiver extending to the ear ("body" type). This type of aid is generally used in cases of extreme hearing loss and often by children needing robust, reliable devices. The largest number of hearing aids in use are light weight hearing aids worn on the side of the head. There are two types available today; one is the "behind-the-ear" aid (the familiar half moon shaped instrument), and the other is an "eyeglass" aid. In both of these models the earphone (receiver) is located in the hearing aid body and sound is directed to the ear canal with tubing terminated with an ear mold.
A hearing aid consists of four basic components; microphone, amplifier, receiver and battery. Each of these components has to be selected or designed to interact with each of the other sections for optimal performance of the hearing aid.
The input transducer or microphone converts acoustic sound pressure into an electrical signal. The microphone is important in determining several performance characteristics of the hearing aid. Among these are frequency response, signal to noise ratio, acoustic sensitivity threshold, linearity, dynamic range, directionality, etc. Before the 1970's the magnetic and crystal microphones were the standards of the industry. The electret condenser microphone has resulted in the development of hearing aids smaller in size with better performance and reliability than was previously possible.
The electret microphone offers a much broader frequency range than the magnetic or piezo-electric type. The size and weight of the moving system in an electret microphone (light diaphragm only) allows designers to achieve better dynamic range (signal to noise and signal to vibration ratios).
Besides the microphone, there are several alternative input transducers by which the signal can be provided to a hearing aid amplifier: a telecoil; infrared detectors; RF receivers; and direct electrical input. The telecoil was introduced as a solution to overcome high background noise in schools, concert halls, churches, etc., in conjunction with a magnetic loop, where the sound source was far away from the hearing aid microphone. This was especially important with early, poor quality microphones, where a distorted electrical signal was already strongly masked with noise generated by hearing aid vibrations and friction of clothing etc., against hearing aid housing. Permanently installed magnetic loops in conjunction with the telecoil permitted extension of both the frequency spectrum and dynamic range of hearing aids. Electro-magnetic interference, portability and cost were major factors limiting the popularity of magnetic loop systems. The two first drawbacks are often overcome by the use of special wireless hearing aids working with infrared or radio transmitters. However, complexity of a wireless input stage makes hearing aids operating on these principles bulky and expensive.
The popularity of the telecoil as a marketable feature and the limited availability of permanent magnetic loops stimulated other applications of the telecoil such as the use with small magnetic sources (coils for television and radio or telephones and recently with portable room loops or neck loops). Different characteristics of various magnetic sources lead to a variety of telecoils having different sensitivity, frequency characteristics or orientation.
The hearing aid with telecoil and/or electrical input would always have a microphone built in. A switch (often called the "T" switch) permits choice of the input depending on what mode of operation is judged by user to be the best in given circumstances. However, the basic input device, a microphone, is not built into a wireless hearing aid. Therefore, a wireless hearing aid's use is limited to special applications only.
The amplifier boosts the level of electrical output of the microphone or alternative input stage. The amplifier must provide high gain, low noise, and good electrical dynamic range. In the early 70's the advent of the integrated circuit profoundly affected the hearing aid. The integrated circuits used in hearing aids provide amplification and permit addition of other functions required of the amplifiers: frequency corrections, automatic gain control, etc. while still providing long battery life. The amplified electrical signal is delivered to an output transducer.
The output transducer or receiver converts the electrical output of the amplifier to sound pressure. The receiver is the most critical component of the hearing aid design. In most designs, the receiver determines the final output, overall acoustic gain, frequency response and dynamic range of the hearing aid. All hearing aid receivers manufactured today are of the magnetic type. The receiver is terminated with an ear mold which either occludes the ear canal or simply maintains the terminating receiver tube in its proper position. The ear mold is not only the most uncomfortable part of the hearing aid, but can also degrade the quality of the hearing aid and its performance. An alternative to a receiver as an output transducer is a bone vibrator transmitting sound directly to the mastoid.
Some wireless hearing aids will have the output transducer substituted with a magnetic loop placed around the user's neck. This means that a hearing aid such as a behind-the-ear type does not have to be removed, but can be used in its telecoil position to pick up signals produced by the neck loop. In spite of the inconvenience of using two devices, this hardware configuration is gaining increasing application because of its efficiency.
The power source of the hearing aid is a battery. The hearing aid compartment must be opened in order to change batteries, thus allowing dirt and moisture into the instrument, and increasing the chance of mechanical problems. Rechargeable batteries have limited capacity and are used mostly in large hearing aids. However, they make it possible to design a sealed hearing aid. Voltage available from the battery is related to its physical size and often limits the linear amplification capability of the hearing aid.
Comprehensive evaluation of hearing aid performance involves some thirty factors. But the most important parameters are frequency response, acoustic gain and saturated output level. For a telephone hearing aid, however, such as the one contemplated by this invention, these parameters do not represent an important challenge.